Lithium secondary batteries have been widely used as a power source of portable devices since its appearance in 1991's. Recently, according to rapid development of electronics, communications, and computer industries, their application is spreading into camcorders, mobile phones, notebook computers, PCs and the like and development therefor is being intensive more and more. As a power source for driving the portable electronic data communication devices, demands for the lithium secondary batteries are increasing day by day. Particularly, researches about a power source for an electric vehicle, wherein an internal-combustion engine and a lithium secondary battery are hybridized, are actively proceeding in America, Japan, Europe and the like.
As a large battery for electric vehicles, a nickel-hydrogen battery, whose development is still in the beginning step, has been used in the view of stability, but the use of a lithium ion battery has been considered in the view of energy density. But the biggest challenge is high cost and safety. Particularly, the anode active materials such as LiCoO2 and LiNiO2, which have already been commercialized, have defects that the crystal structure becomes instable by delithiation during charging, and therefore, thermal stability also becomes very poor.
Small lithium ion secondary batteries now sold on the market use LiCoO2 as an anode active material. The LiCoO2 is an excellent material having stable charge/discharge characteristics, excellent electric conductivity, high stability and even discharge voltage, but because Co has low deposit and is expensive and toxic to human body, development of other anode materials is needed. LiNiO2 having the layered structure like LiCoO2 shows large discharge capacity, but it has not been commercialized yet because it has low cycle life and stability problem at higher temperature and is the most thermally unstable.
In order to improve the problems, there have been many attempts to replace the part of the nickel with transition metal atoms. However, satisfactory results have not been obtained yet. For example, Japanese Laid-Open Patent Publication No. Hei 8-171910 disclosed a method in which Mn and Ni are coprecipitated by adding an alkaline solution into an aqueous solution mixture of Mn and Ni, then lithium hydroxide is added and the resulting mixture is calcined to obtain an anode active material represented by the formula LiNixMn1−xO2(0.7≦x≦0.95).
Recently, Japanese Patent Application No. 2000-227858 disclosed an anode active material prepared by a novel technology about uniformly distributing Mn and Ni compounds in atomic level to form a solid solution, not the technology partly substituting LiNiO2 or LiMnO2 with transition metals. However, due to reactivity of Ni4+, it is difficult to be commercialized and the thermal stability problem of the active material comprising Ni has not been solved yet.
As the most highlighted substitute materials for LiCoO2 having a layered crystal structure, there are Li[Ni1/2Mn1/2]O2 and Li[Ni1/3Co1/3Mn1/3]O2 and the like, in which nickel-manganese and nickel-cobalt-manganese are mixed at a ratio of 1:1 or 1:1:1, respectively. These materials show lower cost, higher capacity and better thermal stability than LiCoO2. However, their high-rate and low-temperature characteristics become lower due to their lower electric conductivity than LiCoO2, and energy density of a battery is not improved in spite of high capacity due to their low tap density. Particularly, due to their low electronic conductivity (J. of Power Sources, 112 (2002) 41-48), their high power property as a hybrid power source for electric vehicles is lower than LiCoO2, LiNiO2 and LiMn2O4.
In conclusion, as lithium transition metal-based oxides having Rm-type layered crystal structure, there are LiCoO2, LiNiO2, LiNi1−xCoxO2, LiNi1−x−yCoxMyO2 (M=Mn, Al, Mg, Ti, Ti1/2—Mg1/2), LiNi1/3Co1/3Mn1/3O2, LiNi1/2Mn1/2O2, LiNixCo1−2xMnxO2, Li1+z[NixCo1−2xMnx]1−zO2 and the like. Generally, in these materials, the composition on the particle surface is identical with the composition inside the particle. In order to have excellent anode performance, the function inside of the anode powder particles and the function on the surface thereof should be different each other. Namely, the composition inside the particle should have many insertion/extraction sites and be structurally stable, but it should minimize reactivity with an electrolyte solution on the particle surface.
There is a surface-coating method as a method changing the surface composition of the anode active material. Generally, it is known that the amount of coating is very small of 1 to 2 weight % or less based on an anode active material, and the coating layer forms a very thin membrane layer of about several nanometers to prevent side reaction with an electrolyte. Or, sometimes, when the heat-treating temperature after coating is too high, a solid solution is formed on the surface of the powder particles and the metal composition is different with the composition inside the particle. In this case, the thickness of the surface layer bound to the coating material is several nanometers or less, and there is a dramatic difference between the coating layer and the particle bulk. Therefore, after long-term use of hundreds of cycles, the effect becomes lower. Further, the effect is halved by incomplete coating, namely, non-uniform distribution of the coating layer on the surface.
In order to correct the defects, lithium transition metal oxides having concentration-gradient of metal composition, which were manufactured by a method of synthesizing a previous inner material, coating a material having different composition outside thereof to prepare a bi-layer, and mixing with lithium salts followed by heat-treating thereof, have been suggested. For example, Korean Patent Publication No. 2005-0083869 disclosed that through a heat-treating process, gradual concentration-gradient of the metal composition could be obtained, but difference in the concentration-gradient was hardly formed at high heat-treating temperature of 850° C. or more due to thermal diffusion of the metal ions. Further, because the powder synthesized by this invention had low tapped density, it was not enough to be used as an anode active material for a lithium secondary battery. Further, in this method, when the lithium transition metal oxide was used as an internal material, reproducibility was reduced due to difficulty in controlling the amount of lithium on the outmost layer. When the lithium is excessively synthesized, lithium carbonates and lithium hydroxides are remained on the surface. Accordingly, a lot of gases are produced at high temperature when assembling a cell and hereby cell cases can be easily swollen, gelation can be easily occurred when mixing the electrode for the cell assembly, and aggregation can be occurred when coating the electrode and hereby surface defect can be generated.